This invention relates to devices for measuring odors, gases with odors, or components with odors contained in a sample gas by using odor sensors which are a kind of gas sensors. Such devices according this invention are useful in a large variety of fields of application such as quality control of foods and spices, quantitative analysis of public nuisance involving unpleasant odors, fire detection from a smell of burning matters, and even police works such as tracking and identification of persons in criminal cases and drug inspection.
Odor sensors are adapted to measure changes in themselves electrically or optically caused by odorous components in air or a sample gas which is supplied as they are adsorbed to their odor-sensitive surface. Odor sensors using semiconducting oxides, electrically conductive polymers, a quartz oscillator and a surface acoustic wave (SAW) device are known. Sensors using conductive polymers make use of the change in the conductivity of the polymer as an odorous component is adsorbed. Sensors using semiconducting oxides make use of changes in the resistance of the semiconducting oxide due to the oxidation-reduction reaction of the odorous components in the sample gas. Sensors with an odor-adsorbing film formed on the surface of a quartz oscillator or a SAW device make use of changes in the frequency of oscillation due to change in weight caused by the adsorption of the odorous components. By using odor sensors of these kinds, odor measuring devices can identify, classify and evaluate a given odor, that is, they can determine, when an unknown odorous substance is given, smell or fragrance of what substance its odor most closely resembles, or how to categorize its odor such as the smell of something burning or that the stench of a rotten matter.
Odor sensors using different materials can detect different compounds. Among odor sensors using odor-sensitive films made of conductive polymers, sensors for the detection of different compounds can be obtained by changing the kind of the polymer or the kind of the dopant used for adjusting the conductivity). In general, it is not that each kind of odor sensor can detect only one kind of compounds but most odor sensors can each respond to a number of substances. For analyzing a mixture of many compounds, therefore, a plurality of odor detectors with different sensitivity characteristics are used and the plurality of detection signals from them are analyzed together.
In general, better results of measurement can be expected if a large number of sensors are used. The detection signals from these many sensors may be directly displayed or a technology called chemometrics may be used to carry out a multivariate analysis to measure the odorous compounds. When many sensors are used, however, there are situations wherein some of them turn out to be inappropriate for the detection of odorous components in certain sample gases such that the results of the measurement become worse even if the number of sensors is increased.
An analysis may be carried out, for example, on the basis of ratios between the absolute values of the levels of individual signals obtained from a plurality of sensors and the signal levels of the plurality of sensors. By such a method of analysis, relatively accurate identification is possible if there is a nearly linear relationship between the concentration of the substance and the output level, as is the case with sensors using conductive polymers. In the case of a semiconducting metallic oxide sensor, however, the relationship is non-linear and the analysis is not simple or easy.
It is therefore an object of this invention to provide a device for measuring gases adapted not to use detection signals from unsuitable sensors such that the accuracy of measurements can be improved.
It is another object of this invention to provide a device for measuring and identifying odors even if sensors with a non-linear response characteristic against concentration are present.
A device for measurement embodying this invention, with which the latter object can be accomplished may be characterized as comprising means for preparing a target gas to be measured from a given sample gas containing an odor component by adjusting concentration of the odor component, a plurality of sensors with different response characteristics for detecting the odor component in the target gas, and a signal processor for analyzing detection signals from the sensors as target gases with different concentrations of this odor component are measured. The odor component is characterized on the basis of an analysis on these detection signals.
For preparing such target gases containing the odor component at different concentrations, the sample gas containing this odor component is passed through a collector tube containing an adsorbent which adsorbs this odor component at normal and subnormal temperatures and desorbs it when heated. After a specified amount of the odor component is thus adsorbed to the adsorbent in the collection tube, the tube is heated and a inert gas is passed through as a carrier gas such that the desorbed odor component is carried to the detectors as a target gas. The concentration of the odor component in such a target gas is controlled by the manner of flow of the inert carrier gas.
The detection signals from the plurality of detectors are analyzed by a suitable method of multivariate analysis such as the principal component analysis. In order to obtain a dependable result from such an analysis, response characteristics of each of the sensors may be analyzed by examining the relationship between the outputted detection signal and the concentration of the odor component in the target gas. Only those of the sensors which show a monotonically varying or linear relationship may be considered trustworthy and only the detection signals from such trustworthy detectors may be used for the analysis.